The applicant proposes to study the interaction between neural and biochemical factors in the control of multi-joint arm posture and movement in monkeys, humans, and patients with motor disturbances. The net stiffness at the hand will be determined as it is perturbed from a resting state: the net restoring forces will be measured as the hand is perturbed from rest at given positions in a horizontal work space. The shape, orientation, and a magnitude of these restoring forces will be measured, referred to herein as the "elastic field." Thus, the relative magnitude of the sping-like and non-springlike forces at the hand will be studied in the following experiments: for the two-joint case (free movement at shoulder and elbow), the variation in the elastic field will be measured as a function of position of the hand in a horizontal workspace. Its variation with an expected external perturbation in force or load will be investigated. The influence of intrinsic biochemical factors on the elastic field will be determined by severing the tendons of the arm muscles in monkeys. Patients who have undergone therapeutic muscle-transposition surgery will also be studied. The relationship between posture and movement will be studied by determining the influence of the preceding movement on the elastic field at the final position. The kinematics and postural control of three-joint arm motion will be studied by repeating the experiments with wrist unbraced. Supraspinal control of posture and movement will be determined by specifying the elastic field in patients with lesions in motor cortex, cerebellum, and basal ganglia. The results of these patient studies will guide the later investigations in monkeys with experimental lesions. The experiments described above will also use EMG recordings. Arm position and kinematics will be measured with analog transducers and digital computers. Analytic and simulation studies will be conducted in conjunction with the experiments to help analyze and interpret results. Mathematical analysis and computer simulations of the combined behavior of the inertial and elastic properties of the musculo-skeletal system will be conducted.